This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A138340 #12 Jan 26 2020 21:03:13
%S A138340 1,-4,-32,-64,256,2048,4096,-16384,-131072,-262144,1048576,8388608,
%T A138340 16777216,-67108864,-536870912,-1073741824,4294967296,34359738368,
%U A138340 68719476736,-274877906944,-2199023255552,-4398046511104,17592186044416
%N A138340 Expansion of (1-8x)/(1-4x+16x^2).
%C A138340 abs(a(n)) = 2^A047267(n).
%H A138340 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (4, -16).
%F A138340 a(n) = 2*4^n(cos(Pi*(n+1)/3) - sqrt(3)*sin(Pi*(n+1))/3).
%F A138340 a(n) = 4^n*Sum_{k=0..n} A121314(n,k)*(-1)^k*3^(n-k). - _Philippe Deléham_, Nov 01 2008
%F A138340 a(n) = A128018(n)*2^n. - _Philippe Deléham_, Nov 14 2008
%F A138340 a(n) = 4*a(n-1) - 16*a(n-2); a(0)=1, a(1)=-4. - _Harvey P. Dale_, Sep 30 2014
%t A138340 CoefficientList[Series[(1-8x)/(1-4x+16x^2),{x,0,30}],x] (* or *) LinearRecurrence[{4,-16},{1,-4},30] (* _Harvey P. Dale_, Sep 30 2014 *)
%K A138340 easy,sign
%O A138340 0,2
%A A138340 _Paul Barry_, Mar 15 2008